This invention relates to the field of signal processing, and in particular, to optical signal processing utilizing coherent detection.
Signal processing typically involves the modulation of a source signal by an information signal to produce a modulated signal. The modulated signal, after transmission and any appropriate processing, is demodulated to recover the information signal, as modified by the processing.
Coherent detection, sometimes called synchronous detection, is a method of detection in which the demodulation employs a reference signal, sometimes called a local oscillator signal, that is in synchronisim with the source signal.
In optical processors, the input information signals are typically time-varying electrical signals which modulate optical source signals to produce optical, modulated signal. The optical, modulated signals are processed and then demodulated to produce electrical output information signals. The modulation converts the electrical signals to optical signals and the demodulation converts the optical signals back to electrical signals. The conversion from electrical to optical signals is undertaken because the desired signal processing can be performed better optically than it can be electrically.
In laboratory environments, coherent-detection optical processors have been known in the prior art. In such processors, coherent light from a laser source is diverted into two paths by means of beam-splitters and mirrors. One path passes the optical source beam through a modulator to modulate the source beam with an appropriate input information signal. The other path from the laser source is transmitted withot modulation to form a coherent reference signal. The reference signal is added to the modulated signal to enable coherent detection. Because the reference signal and the modulated signal travel in different paths, careful alignment of the beam-splitters, mirrors, and other optical elements is required in order to achieve satisfactory coherent detection. Relative displacement between the modulated signal and the reference signal by one-half of a wavelength (for example, less than a micron) can cause complete phase reversals which change the phase of the output information signal by 180 degrees. Because ot the careful alignment and high stability required in prior art coherent-detection optical processors, they have been generally regarded as impractical except in laboratory environments.
Optical processors for real-life cross-correlation have recognized the extreme alignment sensitivity of the modulated signal and the reference signal beams. Vibration of the apparatus or inhomogeneities of the optical mediums usually cause optical path differences which substantially interfere with the desired processing. In order to overcome such problems, collinear heterodyning has been proposed.
Collinear hetrodyning is described, for example, in the article "Collinear Heterodyning and Optical Processors" by Herbert R. Carlton, William T. Maloney and Gerald Meltz, Proceedings of the IEEE, Vol. 57, #5, May 1969, pages 769-775. In that article, a real-time cross-correlator is described in which the reference signal and the modulated signal are collinear and hence are not subject to error-causing optical path differences and other problems created by the use of separate optical paths.
Other optical processors have been known which employ collinear optical processing. Such systems, however, have not provided a satisfactory method of and apparatus for processing wide-bandwidth information signals to provide wide-bandwidth output signals.
In light of the above background, there is a need for a wide-bandwidth, high-resolution stable optical processor employing coherent detection.